1. Field of the Invention
This invention relates to a fuel injection controlling apparatus for reducing NOx amounts in an exhaust gas of a Diesel engine that executes lean combustion.
2. Description of the Related Art
Various proposals have been made in the past to reduce amounts of NOx contained in an exhaust gas of a Diesel engine. For instance, Japanese Patent Laid-Open Nos. 218920/1996 and 358717/1992 describe one of such proposals.
The proposal described in Japanese Patent Laid-Open No. 218920/1996 arranges a NOx absorbing agent that normally absorbs NOx, renders an air-fuel ratio of an exhaust gas rich, and emits and reduces NOx when the built-up amount exceeds a limit. When NOx is to be emitted from the NOx absorbing agent, an engine control circuit lowers an operation air excess ratio. At the same time, this control circuit advances fuel injection timing, increases a fuel amount to be injected into a combustion chamber before ignition from a fuel injection valve, and switches a combustion condition of the engine from a combustion condition mainly of normal diffusion combustion to a combustion condition mainly of pre-mixing combustion. In this way, this technology lowers the air excess ratio without generating smoke, renders the air-fuel ratio of the exhaust gas rich and conducts emission of NOx absorbed by the NOx absorbing agent and reduction-purification.
The technology described in Japanese Patent Laid-Open No. 358717/1992 arranges a catalyst converter for reducing NOx and a lean sensor inside an exhaust passage of a Diesel engine. There are also arranged an inter-cylinder injector for injecting a fuel into a cylinder and an intake system injector for injecting the fuel into an intake system. The inter-cylinder injector injects the fuel into the cylinder during the normal operation. When NOx is emitted from the catalyst converter and is reduced, the amount of the fuel to be injected from the intake system injector is calculated from an engine load and the NOx amount. The intake system injector injects the fuel into the intake system on the basis of the calculation result in addition to fuel injection from the inter-cylinder injector, renders the air-fuel ratio of the exhaust rich and supplies HC required by the catalyst converter for reducing NOx.
However, the constructions described above involve the following problems.
In the construction described in Japanese Patent Laid-Open No. 218920/1996, the fuel amount injected into the combustion chamber from the fuel injection valve before ignition is increased by advancing the fuel injection timing so as to switch the engine combustion from the combustion mainly of diffusion combustion to the combustion mainly of pre-mixing combustion. In other words, the fuel is injected under the state where a piston position is low. In consequence, large amounts of the fuel directly adhere to the inner wall of a cylinder liner and are carbonized to thereby increase soot in oil. Since large amounts of the fuel are injected into the cylinder within a short time, mixing of air and the fuel does not easily become uniform, and fuel consumption gets deteriorated.
In the construction described in Japanese Patent Laid-Open No. 358717/1992, the inter-cylinder injector injects the fuel during the engine operation and diffusion combustion is made. When the catalyst converter requires HC for reducing NOx, the intake system injector further injects the fuel to the intake system. Therefore, large amounts of NOx are emitted even in a low engine load zone, and the scale of the catalyst converter must be increased. In addition, the amount of the fuel injected to the intake system increases and fuel consumption gets deteriorated.
In view of the problems described above, the invention is directed to provide a fuel engine injection controlling apparatus for an engine that decreases the amount of NOx in the exhaust and needs less fuel consumption.
To accomplish the object described above, the first invention of this invention provides a fuel injection controlling apparatus for a Diesel engine including first fuel injection means for supplying a fuel into cylinders and second fuel injection means for supplying the fuel to an intake passage, for conducting lean combustion under a normal operating condition, the fuel injection controlling apparatus comprising: engine load detection means; and a controller for inputting a detection signal from the engine load detection means, and causing the first fuel injection means to inject the fuel into the cylinders when an engine load is in a high load zone and the second fuel injection means to inject the fuel into the intake passage when the engine load is in a low load zone.
According to the first invention, in the Diesel engine including the first fuel injection means provided to the cylinder and the second fuel injection means provided to the intake passage, the second fuel injection means injects the fuel in the low engine load zone. Therefore, pre-mixing uniform combustion can be acquired in the low load zone, and the generation amounts of NOx can be drastically reduced. In the high load zone, on the other hand, the first fuel injection means injects the fuel into the cylinders. Therefore, stable combustion can be acquired.
In the first invention described above, the second invention employs the construction equipped with a controller for causing both of the first and second fuel injection means to inject the fuel when the engine load exists in a predetermined boundary zone between the high load zone and the low load zone.
According to the second invention, the fuel is injected to both of the cylinder and the intake passage in the predetermined boundary zone between the high load zone and the low load zone. Therefore, when the engine load passes by the boundary between the high load zone and the low load zone, a drastic change between combustion by the injection into the cylinder and combustion by injection into the intake passage can be mitigated, the occurrence of torque fluctuation becomes less and a smooth engine operation can be conducted.
In the first or second invention described above, the third invention employs the construction including a NOx absorption/reduction catalyst disposed in an exhaust pipe, for absorbing NOx when an air-fuel ratio of an exhaust gas is lean, and emitting NOx when the air-fuel ratio of the exhaust gas is rich; and a controller for causing the second fuel injection means to inject a predetermined amount of the fuel into the intake passage so that the air-fuel ratio of the exhaust gas attains a value approximate to a theoretical mixing ratio when the NOx absorption/reduction catalyst emits NOx.
The third invention includes the NOx absorption/reduction catalyst in addition to the fuel injection controlling apparatus having a small amount of NOx in the exhaust gas at the low engine load. Therefore, the NOx absorption/reduction catalyst need not be big in size. When the air-fuel ratio of the exhaust gas is rendered rich in the low load zone, the second fuel injection means injects the fuel into the intake passage so that the air-fuel ratio attains a ratio approximate to the theoretical mixing ratio. Since a uniform air-fuel mixture can be obtained and pre-mixing combustion is conducted, the generation amounts of NOx can be reduced, and deterioration of a fuel consumption ratio and abnormal high temperature inside the combustion chamber can be avoided.
In the first to third inventions described above, the fourth invention includes intake throttle means for decreasing the intake air amount or/and exhaust throttle means.
The fourth invention can decrease the intake air amount when intake air is throttled. Therefore, the overall air amount becomes small, and deterioration of the fuel consumption ratio when the air-fuel ratio of the exhaust gas is rendered rich can be further decreased.
In the first to fourth inventions, the fifth invention includes an exhaust gas-recirculation device for mixing the exhaust gas to intake air.
Since the exhaust gas-recirculation device is disposed according to the fifth invention, the air-fuel ratio of the exhaust gas can be rendered rich when the exhaust gas is recirculated. Therefore, the fuel injection amount can be further reduced and combustion can be stabilized.